Fix xtext grammar and reference node in process

[ipa-rwu]

Exmaple can be fund here: ipa-nhg/ros-model-examples#8

[ipa-nhg]

@ipa-rwu Thanks a lot for the PR and the changes. Good job!

Comments:

• This change is wrong 3aa5e54 , they are the specifications of arrays, like DiagnosticStatus[] for https://docs.ros2.org/galactic/api/diagnostic_msgs/msg/DiagnosticArray.html.
• I am not sure about d1ce353 I have to check this at metamodel level.
• There is something wrong with the format for the definition of the message, my plan now (with yaml format) is to have a definition as similar as possible to the .msg files; like:

std_msgs:
 specs:
 msg: ColorRGBA
 	message:
 		float32 r
 		float32 g
 		float32 b
 		float32 a
 msg: Header
 	message:
 		int32 seq 
 		time stamp 
 		string frame_id
std_srvs:
	specs:
	 srv: Trigger
	 	response:
	 		bool sucess
	 		string message

Your examples are quite different -> https://github.com/ipa-nhg/ros-model-examples/pull/8/files#diff-94d4e10765013b88643ad70600a187b9035d5103d84c8c1428eb2476947a1114

I will cherry pick the changes that are fine for now and create new separate PRs to discuss the others.

[ipa-nhg]

PreListElement was defined in another commit, I will remove it from here. This commit is about uncapitalize the definition of the attributes

[ipa-nhg]

This is not needed if the metamodel is correctly defined, see: d1ce353#r113516155

[ipa-nhg]

this can be simplified to:

nodes+=[RosNode| EString]

[ipa-nhg]

The grammar is wrong, this implementation doesn't allow a process with a single node. It should be:

('nodes:' '['nodes+=ComponentRef (',' nodes+=ComponentRef)*']')?

[ipa-nhg]

was this "BEGIN" removed intentionally? why?
The interfaces should be defined under the node.

[ipa-nhg]

This PR can be closed, it is superseded by:

#195 #196 #197